1. Field of the Invention
The present invention relates to a process for preparing aqueous polyurethane dispersions from a mixture of cyclic and non-cyclic diisocyanates using a block copolymer process and to the resulting aqueous polyurethane dispersions.
2. Description of the Prior Art
The production of aqueous polyurethane dispersions is known and described in U.S. Pat. Nos. 4,066,591; 4,092,286; 4,108,814; 4,237,264; 4,203,883 and 4,408,008. The aqueous polyurethanes prepared in these patents are predominantly based on cycloaliphatic diisocyanates. While coatings prepared from these aqueous polyurethanes are suitable for many applications, there is a need for products that have greater flexibility, while retaining their high tensile strength.
It would be expected that aqueous polyurethane dispersions based on non-cyclic diisocyanates, such as 1,6-hexamethylene diisocyanate, would provide greater flexibility. However, attempts to prepare aqueous polyurethane dispersions, which contain hydrophilic carboxylate groups, from this diisocyanate using the well known prepolymer process have not been successful. In this process an NCO prepolymer is prepared in the melt or in an organic solvent and is then dispersed in water and chain extended. Attempts to prepare the NCO prepolymer from 1,6-hexamethylene diisocyanate are problematic for one of two reasons: 1) when the prepolymer is dispersed in water, the viscosity of the dispersion rapidly increases forming a thick paste that cannot be chain extended, or 2) during amine chain extension in the aqueous phase, insoluble urea gel particles are formed that preclude the use of the product for commercial applications.
It is an object of the present invention to provide a process for the preparation of aqueous polyurethane dispersions that can be used to prepare coatings having improved flexibility without substantially affecting the other valuable properties of these coatings, such as high tensile strength and good hydrolytic stability. It is an additional object of the present invention to incorporate large quantities of non-cyclic diisocyanates in the aqueous polyurethanes without suffering from the previously discussed problems associated with prior art processes.
These objects may be achieved with the process according to the present invention for preparing aqueous polyurethane dispersions in which a multi-step process is used to prepare the NCO prepolymer. In the first step of the process a cyclic diisocyanate is reacted with a compound containing ionic or potential ionic groups. Thereafter, the non-cyclic diisocyanate and the remaining isocyanate-reactive compounds are added. Further details concerning this process are set forth hereinafter.
U.S. Pat. No. 4,871,798 discloses a process for reducing the amount of ionic groups needed to prepare stable aqueous polyurethane dispersions by initially reacting a compound containing ionic or potential ionic groups with a diisocyanate prior to the addition of the high molecular weight polyol and other optional isocyanate-reactive components used for the preparation of the NCO prepolymer. However, this patent is directed to improving the dispersibility of the polyurethane as opposed to enabling the production of polyurethane dispersions from non-cyclic diisocyanates. In addition, the patent does not describe reacting the cyclic diisocyanate with the compound containing ionic or potential ionic groups before subsequently incorporating the non-cyclic diisocyanate into the polyurethane in order to overcome the previously discussed difficulties.